Practice, practice, practice! Voiceover is about learning that you will be rejected, and often at that, but that you can never stop trying. Use places like the Voice Acting Club or Voice Acting Alliance to hone your skills with your peers and challenge your boundaries and range.

"How do I voiceover improve" is a very broad question, one I couldn't possibly answer in just 5,000 characters, but you are more than free to add me on Skype or check out those websites I mentioned above and immerse yourself in the community to find out!